Chirp spread spectrum system and method

ABSTRACT

A radio frequency receiver for receiving an interleaved single carrier chirp spread spectrum signal based on a chirp sequence includes: a demodulator configured to receive, demodulate, and digitize a modulated radio frequency (RF) signal to form a digital baseband signal; and a signal decoder configured to: despread the digital baseband signal by at least: buffering signal data into blocks of L samples; multiplying the blocks of data with a conjugate of the chirp sequence to form a result; and performing a Fast Fourier Transform (FFT) on the result; equalize data from the FFT to form N data values, where N is the number of subchannels; transform the N data values into N demodulation symbols via an N-point inverse Fourier transform; and decode the demodulation symbols to form codec symbols.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation application of U.S. patentapplication Ser. No. 14/731,226 (hereafter the '226 Application)entitled “CHIRP SPECTRUM SYSTEM AND METHOD”, filed Jun. 4, 2015(currently pending), which is a continuation application of U.S. patentapplication Ser. No. 13/795,973 (hereafter the '973 Application)entitled “CHIRP SPREAD SPECTRUM SYSTEM AND METHOD”, filed on Mar. 12,2013, now Issued U.S. Pat. No. 9,083,444. The '226 Application and '973Application are both incorporated herein by reference in their entirety.

BACKGROUND

In certain long range data communication and telemetry applications, theexchange of information occurs at a relatively lower data rate. In suchapplications, it can be advantageous to build a low to medium data ratesystem that possesses the characteristics of a widely variable data ratesystem, but with reduced power requirements.

For an RF communication system the power amplifier (PA) is commonly themain contributor to the power consumption of the system. The Peak toAverage Power Ratio (PAPR) of the transmit signal impacts the requiredlinearity and peak transmit power specification of the PA. That is, PAPRdrives average power. By lowering the PAPR of the transmit signal, thepower consumption, cost, and heat dissipation of the PA can be reduced.

There are many solutions for reducing PAPR. Some of these involvepredicting peaks in the filtered transmit signal and reducing the peaksby modifying the pre-filtered transmit signal. These methods introducedistortion and are limited in the amount they reduce peak power.

A different method for reducing PAPR is to construct a transmit signalwaveform that naturally has a low peak to average ratio. For OrthogonalFrequency Division Multiplex modulation, a method for reducing the PAPRis to pre-code the transmit modulation symbols using a FourierTransform. This technique, used in Long Term Evolution (LIE) 4G cellularsystems, essentially gives the multi-carrier OFDM signal thecharacteristics of a much lower PAPR single carrier signal.

Pre-coded OFDM or FDMA does not solve the problem since this type ofmodulation is designed for high rate (>1 bps/Hz) and is not extensibleto lower data rates (<<1 bps/Hz) that operate at very low signal tonoise ratios.

A reduced power system and method for transmission of data at low datarates is needed.

SUMMARY

A radio frequency receiver for receiving an interleaved single carrierchirp spread spectrum signal based on a chirp sequence includes: ademodulator configured to receive, demodulate, and digitize a modulatedradio frequency (RF) signal to form a digital baseband signal; and asignal decoder configured to: despread the digital baseband signal by atleast: buffering signal data into blocks of L samples; multiplying theblocks of data with a conjugate of the chirp sequence to form a result;and performing a Fast Fourier Transform (FFT) on the result; equalizedata from the FFT to form N data values, where N is the number ofsubchannels; transform the N data values into N demodulation symbols viaan N-point inverse Fourier transform; and decode the demodulationsymbols to form codec symbols.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 shows a chirp-based communications system, consistent with anexample embodiment of the invention.

FIGS. 2A and 2B illustrate a transmitter, consistent with an exampleembodiment of the invention.

FIG. 3 shows a method of interleaving symbols, consistent with anexample embodiment of the invention.

FIG. 4 shows a modulated signal, consistent with an example embodimentof the invention.

FIGS. 5A and 5B illustrate a transmitter, consistent with an exampleembodiment of the invention.

FIG. 6 shows a modulated signal, consistent with an example embodimentof the invention.

FIGS. 7 and 8 illustrate transmitters, consistent with exampleembodiments of the invention.

FIG. 9 illustrates a receiver, consistent with an example embodiment ofthe invention.

FIG. 10 illustrates a Fast Fourier Transform and equalizer, consistentwith an example embodiment of the invention.

FIG. 11 illustrates a receiver, consistent with an example embodiment ofthe invention.

DETAILED DESCRIPTION

In the following detailed description of example embodiments of theinvention, reference is made to specific examples by way of drawings andillustrations. These examples are described in sufficient detail toenable those skilled in the art to practice the invention, and serve toillustrate how the invention may be applied to various purposes orembodiments. Other embodiments of the invention exist and are within thescope of the invention, and logical, mechanical, electrical, and otherchanges may be made without departing from the subject or scope of thepresent invention. Features or limitations of various embodiments of theinvention described herein, however essential to the example embodimentsin which they are incorporated, do not limit the invention as a whole,and any reference to the invention, its elements, operation, andapplication do not limit the invention as a whole but serve only todefine these example embodiments. The following detailed descriptiondoes not, therefore, limit the scope of the invention, which is definedonly by the appended claims.

As noted above, it can be advantageous in certain low data rate, longrange data communication and telemetry applications, to build a low tomedium data rate system. It can be particularly advantageous to build alow to medium data rate system that possesses the characteristics of awidely variable data rate system (i.e., high sensitivity, widebandfrequency diversity, and a near constant transmit envelope), but withreduced power requirements.

One such system is shown in FIG. 1. In FIG. 1, chirp spread spectrumtransmission modulation system 100 includes a transmitter 102 and areceiver 104.

In one embodiment, PAPR is reduced via a multi-channel chirp spreadspectrum modulation method that produces a transmit signal with a lowPAPR characteristic. In one such embodiment, system 100 is implementedas a point-to-point system using low cost radios.

In one embodiment, as is shown in FIG. 2a , transmitter 102 is aninterleaved single carrier chirp spread spectrum (Interleaved SC-CSS)transmitter that performs a pre-coding function at 112 based on aDiscrete Fourier Transform (DFT) prior to sub-carrier mapping at 114. Inthe approach shown in FIG. 2a , the pre-coding function is an N pointDiscrete Fourier Transform (DFT). In the embodiment shown, thispre-coding function is added to the transmitter in order to reduce thePeak to Average Power Ratio (PAPR). When N is a power of 2 the DFT maybe efficiently performed using a Fast Fourier Transform (FFT) algorithm.

In the approach shown in FIG. 2a , a block of K information bits ismapped at 110 to a block of N transmit symbols. The symbols may be BPSK,QPSK, QAM, or any other known type of modulation. The block of symbols,represented as a matrix of complex-valued data, are transformed at 112into the frequency domain via an N point DFT. The N frequency domainvalues are then mapped at 114 to the inputs of an L-point iFFT using aninterleaved subcarrier mapping. This maps each coded symbol to anindividual subcarrier. An L point iFFT then transforms the data to thetime domain at 116.

A representative interleaved subcarrier mapping is shown in FIG. 3. Ascan be seen in FIG. 3, in an interleaved subcarrier mapping consecutivesubcarriers are distributed, or interleaved. In one example embodiment,every L/N subcarrier is used as shown in FIG. 3. For interleavedsubcarrier mapping, the transmit symbol transformation may bemathematically represented more simply in the time domain. One suchexample is shown in FIG. 2b below.

In one embodiment, as is shown in FIG. 2a , transmitter 102 implementsinterleaved single carrier chirp spread spectrum (Interleaved SC-CSS)modulation. In this approach, the block of time domain data produced at116 is multiplied by a chirp sequence at 118. This multiplicationspreads each subcarrier across the occupied bandwidth of the system.Each coded symbol is then effectively mapped to an orthogonal chirp timeoffset. The mapping is spaced sufficiently to allow for multipathreception at the receiver without introducing any inter-channelinterference.

The output from 118 is then serialized (via a parallel to serialconverter at 120), up-sampled and low pass filtered at 122, and thenconverted to an analog signal by a digital to analog converter (DAC) at124. The analog signal is then, at 126, converted to an RF signal,amplified, and routed to an antenna for transmission over the air.

If not for the DFT pre-coding the PAPR of transmitter 102 would be highdue to the fact that the transmit signal consists of the sum of multiplepseudo-random orthogonal symbols. With DFT pre-coding, however, thetransmit signal characteristics are like that of a single carriertransmission, resulting in a pre-filtered signal having a constantenvelope, or a PAPR value of one.

The Interleaved SC-CSS transmitter 102 may be represented more simply inthe time domain. This is shown in FIG. 2b . Representing the symboltransformation entirely in the time domain simplifies the processing.The block of N symbols are simply repeated L/N times at 127. At thereceiver, the demodulator is able to recover the symbols by equalizingthe signal in the frequency domain, similar to the uncoded FDMA case.

An example chirp signal that can be used in the transmitter 102 of FIGS.2a and 2b is shown in FIG. 4. FIG. 4 shows a plot of frequency versustime for two symbols using Interleaved SC-CSS modulation. In this figureonly two channels are shown for clarity. Both channels, one labeled 130and the other labeled 132, sweep the full bandwidth using the samediscrete frequencies. In one example embodiment, 32 channels and 1024discrete frequencies are used across the bandwidth F. Channels 130 and132 are orthogonal due to the time offset between them. In addition, insome embodiments, the channels are separated sufficiently to preventmultipath from causing inter-channel interference.

A disadvantage of the method of modulation of FIGS. 2a and 2b is thatthe number of symbols, N, per transmit block, L, is limited due to therisk of inter-channel interference. Therefore, this structure is bestused for low data rates. To achieve higher data rates another type ofchirp spreading is employed. One embodiment of a transmitter 102 usingthis alternate type of spreading and multiplexing is shown in FIGS. 5aand 5b .

In this method the block of coded symbols is repeated and multiplied byan interpolated chirp sequence in the frequency domain. The chirpsequence length is equal to L/N, as is the number of block repetitions.Each coded symbol is therefore spread by a length L/N chirp sequence.Instead of spreading each coded symbol with the same chirp sequence, thechirp signal is interpolated at 128 by a factor of N to improve the PAPRof the time domain signal. This method results in a PAPR of unity forthe pre-filtered transmit signal. With interleaved single carrierorthogonal chirp division multiplexing (Interleaved SC-OCDM), multipathwill not cause inter-channel interference since each channel occupies adifferent set of frequencies. However, frequency offset error at thereceiver will cause inter-channel interference (ICI). Accurate frequencyestimation is therefore required for this method.. For lower data ratesthat operate at low signal to noise ratios, the Interleaved SC-CSSmodulation is a better choice due to the inaccurate frequency offsetestimates that may occur when receiving weak signals. For higher datarates the Interleaved SC-OCDM modulation is a better choice due to thereduced ICI.

In the approach shown in FIG. 5a , a block of K information bits ismapped at 110 to a block of N transmit symbols. The symbols may be BPSK,QPSK, QAM, or any other known type of modulation. The block of symbols,represented as a matrix of complex-valued data, are transformed at 112into the frequency domain via an N point DFT. The block of N symbols inthe frequency domain are simply repeated L/N times at 129 and thenspread at 118 using an interpolated chirp sequence. In the exampleshown, the chirp sequence length is equal to L/N. The chirp sequence isinterpolated at 128.

The L frequency domain values are then converted back into the timedomain at 116 mapped at 114 to the inputs of an L-point iFFT using aninterleaved subcarrier mapping. This maps each coded symbol to anindividual subcarrier. An L point iFFT then transforms the data to thetime domain at 116.

The transmit symbol transformation may also be mathematicallyrepresented in the time domain as shown in FIG. 5 b.

FIG. 6 is a spectral plot of two symbols using Interleaved SC-OCDMmodulation. The figure shows frequency versus time for two channels, oneblack/dark grey (152) the other medium gray/light grey (150), assuming achirp length of 8 samples. The orthogonality of the two channels is dueto the fact the channels operate on separate discrete frequencies.

An example embodiment of Interleaved SC-CSS is shown in FIG. 7. In theexample embodiment shown in FIG. 7, a block of K information bits isencoded at 202. In one such embodiment, forward error correction is usedto provide a more robust communication link and to provide highersensitivity at the receiver 104. The information data is encoded by aconvolutional encoder at 202 and the coded bits are distributed within ablock of data by an interleaver at 204. The coded bits are then mappedto modulation symbols by a QPSK encoder at 206. In both embodiments thenumber of symbols, N, and the transmit block length, L, are powers of 2with N<L. In the OCDM case, since N is a power of 2, the pre-coding isefficiently performed with an N point FFT at 208.

An example embodiment of Interleaved SC-OCDM is shown in FIG. 8. In theexample embodiment shown in FIG. 8, a block of K information bits isencoded at 202. In one such embodiment, forward error correction is usedto provide a more robust communication link and to provide highersensitivity at the receiver 104. The information data is encoded by aconvolutional encoder at 202 and the coded bits are distributed within ablock of data by an interleaver at 204. The coded bits are then mappedto modulation symbols by a QPSK encoder at 206. In both embodiments thenumber of symbols, N, and the transmit block length, L, are powers of 2with N<L. In the OCDM case, since N is a power of 2, the pre-coding isefficiently performed with an N point FFT at 208.

In some example embodiments of transmitter 102 shown in FIGS. 7 and 8, Lis equal to 1024. Due to the large symbol length, no cyclic prefix isadded to each transmission symbol (since the degradation due to ISI issmall).

FIG. 9 shows a diagram of a receiver 104 which may be used to receivethe signal transmitted by the Interleaved SC-CS S transmitter as shownin FIG. 7. An RF demodulator 300 down-converts the RF signal to ananalog baseband signal. The analog signal is converted to a digitalbaseband signal with an Analog to Digital Converter 302. The digitalbaseband signal is then low-pass filtered by a digital filter 304. Thedigital stream of data from the output of the filter is then correlatedwith the conjugate of the chirp signal at 306. This is accomplished bybuffering the data into a block of L samples, multiplying the block ofdata sample by sample with the conjugate of the length-L chirp signal,and then performing at 308, a length L Fourier transform on the data.

The data from the FFT is then equalized at 310 to form N data values,where N is the number of sub-channels. The N values are transformed at320 by an N-point inverse Fourier transform to obtain N demodulationsymbols. A QPSK decoder decodes the demodulation symbols at 322 to formcodec symbols. The codec symbols are then de-interleaved at 324 andconvolutionally decoded at 326 to obtain the recovered information data.In the embodiment shown the underlying modulation scheme is QPSK, butany other modulation may be used such as BPSK or m-ary QAM. In addition,the embodiment shown in FIG. 9 employs convolution coding errorcorrection. The receiver 104 may use no forward error correction, or itmay employ other known error correction codes such as Reed-Solomon, LDPCcodes, or turbo codes.

An L-point FFT 308 that can be used in the SC Interleaved CSS Receiver104 of FIG. 9 is shown in FIG. 10. The figure depicts a K-tap equalizer310 with N sub-channels. The outputs from the FFT are multiplied by thecomplex conjugate of the channel impulse response, C_(k). The results ofthe multiplications are summed to form a complex value for each of thesub-channels.

An SC Interleaved CSS Receiver which employs frequency domaindemodulation is illustrated in FIG. 11. In the example embodiment ofFIG. 11, the filtered, digital baseband signal is first buffered into ablock of L samples and converted into the frequency domain using an Lpoint FFT 312. Conversion into the frequency domain may be desirable inorder to easily filter (at 314) any narrowband interference which mayappear in the pass-band. Following narrowband suppression, the block ofdata is correlated with the chirp signal at 306 by multiplying the datawith the length L chirp signal, then performing an L point FFT at 308.The remaining processing is the same as the time domain demodulator caseas presented in FIG. 9.

The above described chirp spread spectrum modulation methods have theadvantage that they exhibit low transmit signal PAPR, scalable datarates, wideband signal with multipath recoverable at receiver, coherent,high sensitivity acquisition and demodulation, high processing gainsthat are robust to interference and low inter-symbol and inter-channelinterference.

Although specific embodiments have been illustrated and describedherein, it will be appreciated by those of ordinary skill in the artthat any arrangement which is calculated to achieve the same purpose maybe substituted for the specific embodiments shown. The invention may beimplemented in various modules and in hardware, software, and variouscombinations thereof and any combination of the features described inthe examples presented herein is explicitly contemplated as anadditional example embodiment. This application is intended to cover anyadaptations or variations of the example embodiments of the inventiondescribed herein. It is intended that this invention be limited only bythe claims, and the full scope of equivalents thereof.

What is claimed is:
 1. A radio frequency transmitter, comprising: amodulation circuit configured to: map a block of K information bits intoa block of N transmit symbols, wherein N is greater than one; transformthe block of N transmit symbols to a block of N subcarriers in thefrequency domain; interleave the block of N subcarriers in the frequencydomain into a block of L subcarrier; transform the interleavedsubcarriers into a block of time domain data of length L; and multiplythe block of time domain data by a chirp sequence of length L to formchirp spread time domain data for transmission, wherein each subcarriertransformed to the time domain is spread across the occupied bandwidthof the system using the same frequencies to effectively map eachsubcarrier to a chirp time offset.
 2. The transmitter of claim 1,wherein the modulation circuit is configured to map the block of Kinformation bits to a block of N transmit symbols through Quadraturephase-shift keying (QPSK).
 3. The transmitter of claim 1, wherein themodulation circuit is configured to map the block of K information bitsto a block of N transmit symbols through Binary phase-shift keying(BPSK).
 4. The transmitter of claim 1, wherein the modulation circuit isconfigured to map the block of K information bits to a block of Ntransmit symbols through quadrature amplitude modulation (QAM).
 5. Amethod of modulating data for transmission in a system, comprising:mapping a block of K information bits to a block of N transmit symbols,wherein N is greater than one; transforming the block of N transmitsymbols to a block of N subcarriers in the frequency domain;interleaving the block of N subcarriers in the frequency domain into ablock of L subcarriers; transforming the interleaved subcarriers into ablock of time domain data of length L; and multiplying the block of timedomain data by a chirp sequence of length L to form chirp spread timedomain data for transmission, wherein each subcarrier transformed to thetime domain is spread across the occupied bandwidth of the system usingthe same frequencies to effectively map each subcarrier to a chirp timeoffset.
 6. The method of modulating of claim 5, wherein mapping a blockof K information bits to a block of N transmit symbols is performedthrough Quadrature phase-shift keying (QPSK).
 7. The method ofmodulating of claim 5, wherein mapping a block of K information bits toa block of N transmit symbols is performed through Binary phase-shiftkeying (BPSK).
 8. The method of modulating of claim 5, wherein mapping ablock of K information bits to a block of N transmit symbols isperformed through quadrature amplitude modulation (QAM).
 9. A radiofrequency communication system, comprising: a radio frequencytransmitter, wherein the radio frequency transmitter includes amodulation circuit and an RF modulator, wherein the radio frequencytransmitter is configured to: map a block of K information bits into ablock of N transmit symbols, wherein N is greater than one; transformthe block of N transmit symbols to a block of N subcarriers in thefrequency domain; interleave the block of N subcarriers in the frequencydomain into a block of L subcarrier; transform the interleavedsubcarriers into a block of time domain data of length L; and multiplythe block of time domain data by a chirp sequence of length L to formchirp spread time domain data for transmission, wherein each subcarriertransformed to the time domain is spread across the occupied bandwidthof the system using the same frequencies to effectively map eachsubcarrier to a chirp time offset; and a radio frequency receiver,wherein the radio frequency receiver includes a demodulator and a signaldecoder, wherein the radio frequency receiver is configured to: receive,demodulate, and digitize a modulated radio frequency signal to form adigital baseband signal, and recover a signal modulated usinginterleaved single carrier chirp spread spectrum modulation.
 10. Thesystem of claim 9, wherein the radio frequency transmitter is configuredto map the block of K information bits to a block of N transmit symbolsthrough Quadrature phase-shift keying (QPSK).
 11. The system of claim 9,wherein the radio frequency transmitter is configured to map the blockof K information bits to a block of N transmit symbols through Binaryphase-shift keying (BPSK).
 12. The system of claim 9, wherein the radiofrequency transmitter is configured to map the block of K informationbits to a block of N transmit symbols through quadrature amplitudemodulation (QAM).